The present invention relates to heat exchangers. More specifically, the present invention relates to oxidation protection of titanium-based heat exchangers, with improved fatigue properties.
Certain heat exchangers used in aircraft environmental control systems are exposed to temperatures exceeding 400° C. These heat exchangers are typically made of stainless steel or nickel based alloys such as Inconnel, which can withstand the high temperatures.
It would be desirable to use titanium heat exchangers instead of stainless steel heat exchangers. Titanium and its alloys have a lightweight and can provide a weight reduction of up to 40% over comparable stainless steel heat exchangers. The weight reduction results in better fuel efficiency and lower aircraft operating costs.
However, titanium is not used for high temperature heat exchanger applications because the titanium exhibits a propensity to rapidly oxidize, (over a couple of hours), at the required operating temperatures. Oxidation of titanium results in a reduction in ductility and then strength, and a deterioration in structural integrity. Repeated thermal cycling at temperatures between ambient temperature and around 400° C. (and higher) causes the titanium to crack. Cross contamination of fluids can occur and lead to life-threatening conditions.
Accordingly, it has been proposed to provide an oxidation protective coating to the exposed titanium-based surfaces of the heat exchanger. The coating may be a titanium aluminide coating. The coatings and thermal cycles are compatible with the titanium alloy and also the brazes used to join the Ti. It also provides protection to the braze, which can be more prone to oxidation than the Ti. Oxidation of the braze would enbrittel the joint and lead to oxidation of the Ti. Not only does the titanium aluminide provide oxidation protection, but it is able to withstand the different thermal stresses due to thermal cycling set up by either titanium or a braze clad titanium, because of their good bonding to the base titanium and braze plus their high strength. The braze and titanium have different coefficients of thermal expansion so locally at the junction between braze clad and titanium the coating may be subject to high strains and stresses. In addition, the coatings do not reduce the heat exchanger efficiency by reducing the gas flow through the passageways. Neither do they reduce the heat transfer though the titanium; i.e. they do not thermally insulate the titanium that they protect from oxidation.
Moreover, the coatings maintain adequate strength and ductility in the titanium, which allows the heat exchanger to handle structural forces occurring in high temperature heat transfer applications.
In many vehicles and especially in an aircraft there are high pressures and temperature cycling and vibrations from the surrounding environment (e.g., the aircraft engine on which the heat exchanger is attached). The protective coating must therefore also maintain adequate fatigue properties in the material it is protecting. It is however well known that most coatings reduce the fatigue life of material they are protecting. For instance, anodizing of aluminum is a well-known method of protecting aluminum, but it also reduces the fatigue life of the aluminum. This is usually referred to as the fatigue debit. For most coatings there is a need and desire to reduce the magnitude of this fatigue debit and ideally to eliminate it.
The TiAl protective coatings for titanium also have a fatigue debit and as would be expected there is a desire to reduce this debit. Especially as aircraft heat exchangers are usually used in an environment, which is subjected to high fatigue loads, there is a need therefore to improve the fatigue characteristic of the coating and reduce the debit it has on the fatigue properties of the Ti.